It is recognized that absorption cooling can be an effective means for using heat output from a turbine to provide heating and/or cooling to condition the air of a building, and a number of absorption cooling systems currently exist to provide a combination of cooling, heating and power. Such systems are known as tri-generation systems. As shown in FIG. 1, an exemplary tri-generation system 100 includes a chilled water circuit 110, a cooling water circuit 120, a hot water circuit 130, and a circuit 140 for receiving exhaust energy from a turbine-generator 150. This exemplary system also includes an internal refrigerant loop 160 for transferring heat among the cooling water 120, the chilled water 110, and the hot water 130 circuits.
In attempting to provide a control for absorption chillers such as the tri-generation system of FIG. 1, those skilled in the art have implemented a set of control valves so as to modulate flow rates in the system's various circuits. For example, in the illustrated system, energy input valve 141 can be used to control the heat input to the system by controlling the rate of flow of turbine exhaust gas in the exhaust gas circuit. Similarly, hot water valve 131 can be used to control the rate at which heat from the exhaust gas circuit is transferred to the hot water circuit (i.e., the heating loop) by modulating the rate of flow of refrigerant through the heating loop. Finally, chilled water valve 121 can be used to control the rate at which heat is extracted from the chilled water circuit and transferred to the cooling water circuit by modulating the rate of flow of refrigerant through the cooling loop. When the system is controlled properly, and the positions of control valves 121, 131, and 141 are set properly, all system constraints would be satisfied. Thus, the cooling capacity of the system would meet a desired cooling setpoint, the heating capacity of the system would simultaneously meet a desired heating setpoint, and the generator turbine would be producing a desired power output.
Unfortunately, however, prior attempts have failed to provide an automated control that can reliably meet all of these constraints, simultaneously. As a result, prior art controls for such a tri-generation systems typically require a user to preselect a mode of operation for the control. For example, a user may preselect whether the control is to operate in a cooling mode or in a heating mode. In an exemplary cooling mode, a conventional control may modulate the position of energy input valve 141 based on only a prescribed chill water set point such that the adjustment of the position of energy input valve 141 depends only upon the deviation of produced cooling capacity from the demanded cooling setpoint. Accordingly, the position of energy input valve 141 is not affected by changes to hot water temperature. Similarly, in a conventional control operating in heating mode, the modulation of energy input valve 141 may depend only upon the deviation of produced heating capacity from the demanded hot water set point without regard for its effect on chilled water temperature.
Unfortunately, a substantial drawback of conventional tri-generation systems is that they must typically be pre-set so as to be controlled, or to operate, in either a cooling mode or a heating mode, and the task of switching between modes can be problematic. The necessity to pre-select either a heating control mode or a cooling control mode has been found to be impractical when and where switching between modes may be required one or more times within a single day—such as in moderate climates or during moderate seasons. Accordingly, a demand exists for an absorption chiller that can simultaneously (i.e., without switching between two mutually-exclusive modes such as a cooling-only mode and a heating-only mode) be controlled or operated in both heating mode and cooling mode, i.e., so as to simultaneously produce desired levels of both heating and cooling.
Therefore, in addition to the simplified approach described above, a number of attempts have been made to better accommodate concurrent demand for both heating and cooling. For example, previous attempts to control a tri-generation system include a hierarchy of control modes. One such control system includes a so-called priority-override control and a separate capacity control. In accordance with such control schemes, a priority-override controller may manipulate hot water valve 131 and chilled water valve 121 according to whether the system is pre-set for heating priority or cooling priority.
In cooling priority, when cooling capacity is less than cooling setpoint minus cooling override, cooling priority override is active. In accordance with this scheme, control error to be minimized is the difference between actual cooling capacity and desired cooling, or setpoint. The control modulates the position of chilled water valve 121 so as to reduce that control error, i.e., such that the actual cooling capacity meets the setpoint. At the same time, the control may modulate the position of hot water valve 131 in a compensating manner. In a corresponding heating priority mode, when heating capacity is less than heating setpoint minus heating override, heating priority override may be activated. Control error is heating capacity deviation from the heating setpoint, and the control opens or closes hot water valve 131 while adjusting the position of chilled water valve 121.
Alternatively, a tri-generation system may be controlled using a capacity control scheme. In a capacity control, hot water valve 131 is used to minimize the deviation of heating capacity deviation from a heating setpoint while chilled water valve 121 is adjusted so as to maintain sufficiently high hot water outlet temperature.
Nonetheless, while these and other control schemes have provided some improvements in the control of tri-generation systems, control instability remains a persistent issue, and attempts to simultaneously meet a demanded level of cooling and a demanded level of heating, have caused controls to experience difficulty finding appropriate positions for all three control valves. For example, when cooling and heating loops are controlled simultaneously, even when one mode is given priority over another mode, controls have been found to repeatedly switch between modes, such as the above-described priority override control and capacity control modes, resulting in control instability. This occurs, for example, when cooling capacity is greater than or equal to the difference between cooling setpoint and cooling override in cooling priority mode, or when heating capacity is greater than or equal to the difference between heating setpoint and heating override in heating priority mode, then control logic switches to capacity control where no priority override is active. As a result, both hot water temperature and chilled water temperature may oscillate in an unstable and unacceptable manner such that modulation of energy input valve 141 may be unable to continuously accommodate both cooling and heating demand. Put another way, using prior art control systems, the position of energy input valve 141 can only be effectively controlled based on either cooling or heating setpoint with hot water valve 131 and chilled water valve 121 being regulated based on heating and cooling setpoint—independently.
Thus, prior art controllers for simultaneous heating and cooling exhibit discontinuous behavior, with the discontinuity resulting from repeated switching (i.e., control oscillation) between control modes such as priority override and capacity control modes. It has also been discovered that independent control of hot water valve 131 and chilled water valve 121 in capacity control mode can result in oscillation due to strong coupling between the heating and cooling loops and differences between the dynamic response characteristics inherent in heating and cooling loops. As a result, prior art control schemes can be complicated and difficult to service in the field.
Accordingly, a need exists for an improved system and method for controlling an absorption chiller configured to simultaneously provide cooling and heating. More specifically, a need exists for an improved system and method for controlling an absorption chiller that can meet a priority cooling or heating demand while maintaining chilled water and hot water outlet temperatures and their associated control valves stable automatically—without violating chiller safe operating constraints.